The present invention relates generally to aircraft propulsion, and more specifically to aircraft engines incorporating an open rotor for propulsion.
It is well-known to provide aircraft propulsion using open rotors, defined generally as an apparatus including a rotatable hub carrying an array of airfoils which are configured to produce thrust, and which are not surrounded by an outer shroud.
One type of open rotor is a conventional propeller. These are mainly used for low-speed applications and generally become inefficient at high subsonic speeds.
There is an interest in using open rotors to propel higher-speed vehicles using blades optimized therefor; these types of open rotor are often referred to as “prop fans” or “unducted fans”. In high-speed subsonic flight, the combination of rotor rotational speed, blade radius, and flight speed results in blade speeds that are supersonic through surrounding air, particularly at the outer span. When the flow approaching a blade leading edge is supersonic in the rotor frame-of-reference, and if leading edge sweep along the blade span is relatively low, then a bow shock wave (or simply “bow shock”) will exist upstream of the leading edge. Open rotor bow shocks propagate away from the rotor and potentially generate noise, as well as efficiency losses.
Accordingly, in the prior art, it is generally accepted practice that high-speed open rotor blades must use thin leading edges, to minimize the size, strength, and effect of bow shock waves. However, one problem with blades using thin leading edges is that they do not operate efficiently at large positive and negative angles of blade airflow incidence.